The present invention relates to transducers for generating an electrical signal in response to a sensed vibration, and is particularly suitable for application as a sensor to detect the vibration that creates the relatively high frequency sound generated in an engine as a result of improper or premature combustion, often referred to as "knocking" or "pinging". Such knock sensors are used for providing an electrical signal to a microcomputer employed for controlling engine spark timing and electronically controlled fuel injection. Typically in such engine applications, when a signal is received by the microcomputer from the knock sensor, the ignition spark timing is appropriately retarded to prevent continuation of the knocking condition.
Heretofore, vibration transducers employed for sensing engine knocking or knock sensors have employed a centrally supported overhanging metal diaphragm having piezoelectric material bonded to the central portion of the diaphragm for providing an electrical signal in response to resonant flexure of the diaphragm upon vibration of a preselected frequency being imparted to the diaphragm mounting structure. Knock sensors of the aforesaid construction typically provide a high "Q" or a narrow band width displayed in graph of sensor output versus frequency, with the band width being a portion of the graph on both sides of the resonant frequency.
Typically, it has been though desirable to provide a relatively high "Q" for knock sensors in order to have a significant electrical signal output at the resonant frequency to facilitate detection, with a substantially lower signal at frequencies slightly adjacent the resonant frequencies. However, it has been found that a knock sensor having a relatively high "Q" or narrow band width exhibits the characteristic of continuing to provide an electrical signal output upon cessation of the vibration input for a significant duration of time thereafter. This slow output delay phenomenon of the sensor is sometimes referred to as "ringing". This phenomenon or disadvantage of high "Q" knock sensors tends to limit the sampling interval of the sensor. This limitation becomes critical when the engine is operated at a high enough RPM when the interval between firing in the cylinders is of an order of magnitude equal the time required for the sensors to dampen out. Problems have been encountered in certain frequency bands with known knock sensors of the aforesaid type wherein the high "Q" or signal gain properties of the sensor have extended the period of ringing sufficient to prevent the knock sensor from responding adequately at engine RPMs encountered at the upper limit of engine operating speed in excess of 5000 RPM. A four cylinder, four cycle engine running at 5000 RPM fires once each six milliseconds.
Heretofore, attempts have been made to improve the performance of such known transducers by providing an annular convolution or rounded ring-groove semicircular in transverse section in the metal disc.
Thus, it has been desired to provide a vibration transducer for engine knock detection applications which has a high signal output upon detection of acoustic vibration resulting from engine knock, and yet provides rapid enough damping to permit the knock sensor to be effective at high engine RPMs.